People who drink or use drugs to calm anxious nerves are at increased risk of developing full-blown substance abuse problems later on, according to a study.

The work by Canadian researchers writing in the Archives of General Psychiatry, which followed close to 35,000 people, is one of the first to try and answer a long-standing question: do anxiety-ridden people self-medicate because they are substance abusers, or do they become abusers because they self-medicate?

"Self-medication in anxiety disorders confers substantial risk of incident substance abuse disorders," wrote lead researcher Jennifer Robinson at the University of Manitoba in Winnipeg, and colleagues.

The group tapped into a national US survey of drinking problems and mental illness that followed subjects over three years and included interviews.

They found that of those who had anxiety disorder at the outset of the study and said they self-medicated with alcohol, 13 percent developed alcoholism - compared to only about 5 percent of respondents who didn't self-medicate.

After taking income, age and other factors into consideration, self-medicating people had 2.5 to 5 times the odds of becoming dependent on alcohol or drugs compared to people who followed their prescription.

In theory, a person who self-medicates could be a budding drug abuser without the interviewer having spotted it, so the findings aren't conclusive.

But Robinson and her colleagues said their study bolsters the hypothesis that self-medication leads to substance abuse.

They also found that people who self-medicate with alcohol were three times as likely to develop social phobia - although it was possible that those people had some degree of phobia from the start, and their substance use fueled it.

"Another possibility is that the social unacceptability of substance use may create a desire to avoid social contact in those who actively use other drugs," they wrote.